CHEM 162, Winter 2010
Lab 1: Molecular Structure and VSEPR
Monday, January 11

Objectives:

  • To discover systematic ways of determining the shapes of molecules using VSEPR theory
  • To learn to determine molecular polarity based on shape and bond polarity

Procedures:

  1. Draw the Lewis dot structures for each molecule in the following list.
  2. Build a model of the first molecule. Use a stick with no ball on the end to represent a lone pair (you may wish to use the purple sticks to represent lone pairs). Be sure to pick a ball with holes placed at the appropriate angles to match the bond angles. Build the molecule so that all “parts” (the lone pairs and atoms attached to the central atom) are as far away as possible from each other. Draw a picture of this model in column 2. (If the molecule has no lone pairs, leave column 2 blank.)
  3. Now, take out any lone pairs (so that your model only has atoms and bonds). This shows the “molecular geometry” of the molecule. Draw a picture of this model in column 3. (There should be no blanks in column 3.)
  4. In column 4, write the name of the geometry of the molecule (linear, bent, trigonal planar, pyramidal or tetrahedral). Refer to Section 10.2 in your textbook.
  5. In column 5, write the approximate angle between each bond/lone pair in the molecule.
  6. In column 6, write whether the molecule is polar or nonpolar.
  7. Repeat steps 1-5 for each molecule in the table.

Post-Lab Questions (Due on Wednesday, January 20, 2010):

1. Briefly summarize the VSEPR theory and its use.

2. Explain why two compounds with the same generic formula, NH3 and BH3, have different molecular geometries.

3. Explain why the structures of NH3 and PH3are so similar?

4. Although PH3 and BH3 both contain bonds of similar polarity, one molecule is polar while the other is nonpolar. Explain.

5. Heavier molecules tend to have higher boiling points however, NH3 has a significantly higher boiling point than PH3. Why?

6. When faced with several resonance structures, how does one select the “best” structure.

Data Tables (to handed in with above questions on Wednesday, January 20)

Column 1

Lewis Dot Structure /

Column 2

Sketch (with lone pairs) /

Column 3

Sketch (no lone pairs) /

Column 4

Molecular Geometry /

Column 5

Bond Angles /

Column 6

Polar or Nonpolar
BeH2
BH3
IF3
PH3
CH2S

Column 1

Lewis Dot Structure /

Column 2

Sketch (with lone pairs) /

Column 3

Sketch (no lone pairs) /

Column 4

Molecular Geometry /

Column 5

Bond Angles /

Column 6

Polar or Nonpolar
CO2
SiH4
XeF5+
NH3
PO33-

1

Edited by Nick Buker 12/23/08